In mammals, loss of hair cells in the cochlea is permanent. When noise trauma or ototoxic produce cell loss, the anatomical and functional changes remain for life. By contrast, avian cochlear hair cells along with support cells and the tectorial membrane have been shown to regenerate. While this is an finding with potentially major importance, the awareness that the phenomenon is more complicated initial impression has been growing with recent observations that the number of cochlear ganglion cell decrease hair cell regeneration, and that the functional capacity of the auditory system may not fully recover even though nearly all of the hair cells have regenerated. These findings has given special impetus to determining the functional and morphological condition at different levels in the avian auditory system which regenerated hair cells. In the present project, we will approach this issue from the joint perspectives of behavioral audiometry, electrophysiology, and anatomy in the chicken after acoustic overstimulation. The ultimate criterion for assessing the full extent of functional recovery is the functioning of the whole tasks with auditory cues, which behavioral psychoacoustic attempts to evaluate. We will measure pure tone thresholds, psychophysical tuning curves and gap detection after hair cells and support cells have been damaged and allowed to regenerate. All information reaching the higher levels of the auditory system must ass through the cochlear ganglion, and cells in the cochlear ganglion are most intimately related to the cochlear hair cells. For this reason, whole nerve potentials and single unit recordings from cochlear ganglion neuron will be studied to examine the functional status of regenerated hair cells and auditory nerve. Some will be physiologically characterized and retrogradely labeled in order to determine which hair cells innervate along the basilar papilla. Anatomical evaluation of cochlea, cochlear ganglion and cochlear n will help to relate any functional defect to residual structural changes. Finally, the expression of the gi associated protein GAP-43 and the incorporation of bromodeoxyuridine (BrdU), a marker for cell proliferation, will be studied to evaluate the possibility of neural plasticity in the cochlear ganglion and cochlear nucleus associated with hair cell regeneration.